1. Field of the Invention
The present invention relates to a speech recognition system and a standard pattern preparation system for preparing standard patterns to be used for the speech recognition process by the speech recognition system as well as a method of preparing the standard patterns and a computer program for preparing the standard patterns, and more particularly to a speech recognition system for recognizing a narrow-band frequency speech such as a telephone speech recognition.
2. Description of the Related Art
FIG. 1 is a block diagram illustrative of the conventional speech recognition system. The conventional speech recognition system includes a characteristic extraction unit 100 and a pattern reference unit 103. The characteristic extraction unit 100 receives an input of a voice 105 and converts the voice into a characteristic vector time series. The pattern reference unit 103 receives the characteristic vector time series and compares the characteristic vector time series with a standard pattern 104 for the speech recognition before the pattern reference unit 103 outputs a speech recognition result 106. This conventional speech recognition system is addressed in 1995, entitled xe2x80x9cthe fundamentals of the voice recognitionxe2x80x9d NTT Advanced Technology.
A melcepstrum characteristic extraction may be available for the characteristic extraction unit 100. The characteristic extraction unit 100 further includes a power spectrum calculation unit 101 for calculating a power spectrum in a short term of the input voice 105 and a melcepstrum calculation unit 102 receiving the power spectrum from the power spectrum calculation unit 101 and performing a mel conversion and a cosine conversion of a logarithm of the power spectrum, thereby extracting a melcepstrum characteristic quantity.
FIG. 2 is a block diagram illustrative of a conventional standard pattern preparation system. The conventional standard pattern preparation system prepares the above described standard pattern 104 to be referred by the above described conventional speech recognition system shown in FIG. 1. The conventional standard pattern preparation system includes a characteristic extraction unit 200 and a standard pattern preparation unit 204. The characteristic extraction unit 200 further includes a power spectrum calculation unit 201 for calculating a power spectrum in a short term of a learning voice signal from a learning voice storing unit 203, and a melcepstrum calculation unit 202 receiving the power spectrum from the power spectrum calculation unit 201 and performing a mel conversion and a cosine conversion of a logarithm of the power spectrum, thereby extracting a melcepstrum characteristic quantity.
The standard pattern preparation unit 204 receives the melcepstrum characteristic quantity from the melcepstrum calculation unit 202 and prepares a standard pattern. The standard pattern is stored in a standard pattern storing unit 205.
With reference again to FIG. 1 the process for recognition of the narrow band frequency voice such as the telephone voice by the conventional speech recognition system will be described.
The telephone voice has a narrow frequency band, and is likely to receive a substantive influence by noises, for which reason it is generally difficult to recognize the voice. The frequency band of the telephone voice is ranged from 300 Hz to 3400 Hz. A first formant of the vowel or the primary characteristic frequency region is important for the speech recognition. This first formant of the vowel or the primary characteristic frequency region exists under 300 Hz, depending on a speaker. In this case, the voice signal entered from the telephone terminal may be free of the first formant of the vowel or the primary characteristic frequency region under 300 Hz.
A frequency range of the friction noise may often be over 3000 Hz. In this case, the voice signal entered from the telephone terminal may be free of the friction noise.
The restriction on the frequency band causes that the recognition of the telephone voice with the narrow frequency band is lower in accuracy than the recognition of the microphone voice with a wide frequency band.
Japanese laid-open patent publication No. 2000-250577 discloses the following conventional technique for improving the frequency characteristic of the voice with the narrow frequency band entered from the microphone. This conventional technique prevents any lack of the voice information and also improves the speech recognition characteristic in the presence of the noises. A characteristic vector is selected by a first code book from a voice input pattern as received by a second voice receiver. A correction vector is selected from a second code book in correspondence with the index of the selected vector. Both the characteristic vector and the correction vector are then added to presume the characteristic vector of the voice received by the second receiver which ensures a higher voice-receiving sensitivity in a wide frequency band than the first receiver.
In the above circumstances, the development of a novel speech recognition system is desirable.
Accordingly, it is an object of the present invention to provide a novel recognition system free from the above problems.
It is a further object of the present invention to provide a novel recognition system exhibiting such a high performance of speech recognition in a narrow frequency band as closely to the performance in the narrow frequency band.
It is a still further object of the present invention to provide a novel standard pattern preparation system for preparing standard patterns to be used for the speech recognition process by the speech recognition system free from the above problems.
It is yet a further object of the present invention to provide a novel standard pattern preparation system for preparing standard patterns to be used for the speech recognition process by the speech recognition system exhibiting such a high performance of speech recognition in a narrow frequency band as closely to the performance in the narrow frequency band.
It is yet a further object of the present invention to provide a method of preparing the standard patterns free from the above problems.
It is yet a further object of the present invention to provide a method of preparing the standard patterns exhibiting such a high performance of speech recognition in a narrow frequency band as closely to the performance in the narrow frequency band.
It is yet a further object of the present invention to provide a computer program for preparing the standard patterns free from the above problems.
It is yet a further object of the present invention to provide a computer program for preparing the standard patterns exhibiting such a high performance of speech recognition in a narrow frequency band as closely to the performance in the narrow frequency band.
The present invention provides a speech recognition system for recognizing an input voice of a narrow frequency band. The speech recognition system includes: a frequency band converting unit for converting the input voice of the narrow frequency band into a pseudo voice of a wide frequency band which covers an entirety of the narrow frequency band and which is wider than the narrow frequency band.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.